Photographic lenses are generally designed for the best aberrational correction at an infinite focusing distance, so that when focusing is made upon finite distances--by the global translation of the whole lens system--aberrational correction is degraded, especially in the case where the lens is of high relative aperture.
It has been suggested to combime a global translation of the whole lens system with an internal relative translation of one of its components in order to balance aberrational correction, or to translate only several components.